weno ya a la desesperada he hexo esto xD.
this.setTitle( title );
this.setSize( WIDTH, HEIGTH );
this.setLocationRelativeTo( null );
JButton boton1 = new JButton( "Aceptar" );
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
SpringLayout sl = new SpringLayout();
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
JPanel panel = new JPanel( sl );
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
panel.add(boton1);
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
sl.putConstraint( SpringLayout.WEST, boton1, 60, SpringLayout.WEST, panel );
sl.putConstraint( SpringLayout.NORTH, boton1, 40, SpringLayout.NORTH, panel );
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
Container contentPane = this.getContentPane();
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
contentPane.add( panel );
boton1.setSize(240,240);
boton1.updateUI();
boton1.repaint();
se lo he puesto en todas las posiciones y sigue sin ir :(